Trac #30228: Declare Sections with Name only

At this stage we have the following difference between sections and
tensor fields:

{{{
sage: M = Manifold(2, 'M', start_index=1)
....: X.<x,y> = M.chart()
....: E = M.vector_bundle(2, 'E')
....: e = E.local_frame('e')
....: v = M.vector_field('v')
....: s = E.section('s')
Traceback (most recent call last)
...
IndexError: string index out of range
}}}

This simply comes from the fact that the method `section` does not like
pure strings as input, in contrast to `vector_field` or `tensor`.

URL: https://trac.sagemath.org/30228
Reported by: gh-mjungmath
Ticket author(s): Michael Jung
Reviewer(s): Travis Scrimshaw

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=eac827f97c1258cf6524a81084172a43bf7c4680
-md5=bf07fb027bc4c586edf10132298fa363
-cksum=546608769
+sha1=9d04a7066f0c543084ac8249cb280c86d2bf867d
+md5=ac1516d0c732111da045ff977b68230c
+cksum=2699179254

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-e41d396f5d30996ae515399b28675a60e97a1996
+ddd5258c401e17c0ecfb66bdf0b294133abbb4b9

src/sage/manifolds/section.py

@@ -465,6 +465,9 @@ def _init_components(self, *comp, **kwargs):
                     # frame is actually a pair (frame, chart):
                     frame, chart = frame
                 self.add_comp(frame)[:, chart] = components
+        elif isinstance(comp0, str):
+            # For consistency with tensor fields:
+            self.set_name(comp0)
         else:
             if hasattr(comp0, '__getitem__'):
                 # comp0 is a list/vector of components